In offshore petroleum operations, an offshore platform comprising a trussed steel framework substructure secured to the seafloor with an above-water deck mounted on top is commonly used to drill for and produce oil and gas. The trussed steel framework substructure is commonly referred to as a "jacket".
A traditional approach to constructing an offshore platform is to lift individual components of a deck structure onto a preinstalled jacket with a barge-mounted crane and then to interconnect the components and the jacket. This approach can be quite expensive due to the extensive offshore construction required. Offshore construction costs can be high due to the need for special offshore construction vessels and because of downtime caused by such uncontrollable factors as inclement weather and rough seas. In the case of very large platforms, or platforms located in remote areas, offshore construction may require many months to complete.
In an alternate approach to offshore platform construction, known as the "integrated deck approach", a one-piece deck structure or "integrated deck" is used, with most or all components being integrated at an onshore construction yard. The integrated deck is then transported by a vessel such as a barge to the location of a preinstalled jacket, where the integrated deck and the jacket are mated. By using the integrated deck approach, offshore construction time can be greatly reduced. This not only substantially reduces offshore construction costs, but makes the integrated deck approach attractive for offshore areas having short windows of time suitable for construction due to periods of inclement weather or rough seas.
Typically, an integrated deck is mated to a preinstalled jacket either by the use of cranes on offshore crane vessels or by lowering the deck onto the jacket using a float-on deck setting procedure. The second method involves mounting the deck onto a barge, maneuvering the barge into a slot in the jacket framework, properly aligning the barge/deck combination with the jacket, then lowering the deck until it engages the jacket structure. Typically, the deck is lowered by ballasting the barge; however, alternatively, a deck-lowering mechanism can be incorporated into the barge. The deck load is then transferred to the jacket and the barge is disconnected from the deck and removed from the jacket slot. Such a procedure is described in greater detail in U.S. Pat. No. 4,242,011 (Karsan et al.).
Major complications can arise during the integrated deck/jacket mating procedure, due in large part to significant wave-induced relative vertical motion between the barge-mounted integrated deck and the bottom-founded jacket. The shock of wave-induced contact between the deck and jacket can cause appreciable damage to both structures, resulting in delays in completing construction of the offshore platform and in extra expense to repair or replace any damaged equipment.
Attempts to accommodate the wave-induced motion are described in various U.S. patents. For example, U.S. Pat. No. 5,219,451 (Datta et al.) describes an integrated deck provided with shock absorbing stabbing tip assemblies for engagement with the upstanding columns of a jacket substructure. The stabbing tip assemblies include coaxially arranged locating pins which are dropped into position to restrict lateral movement of the deck relative to the upstanding columns and annular resilient collars for transferring vertical and lateral shock loads between the columns and the deck. U.S. Pat. No. 4,930,938 (Rawstron et al.) describes a deck-to-jacket mating assembly fitted with primary and secondary load transfer devices, wherein both devices utilize compression spring systems for transferring vertical and lateral shock loads between the columns and the deck. U.S. Pat. No. 4,848,967 (Weyler) describes a deck to jacket load transfer system which utilizes a non-linear spring system for a soft response at initial impact which transitions to a stiff response. U.S. Pat. No. 4,655,641 (Weyler) describes the use of a stabbing pin assembly which accommodates for wave-induced lateral movement by use of cantilevered springs which are received by receiving members in the jacket structure. Each receiving member is secured in a manner which permits it to tilt in response to lateral forces applied to the receiving member above its bottom end, but which does not permit the receiving member to tilt in response to lateral forces applied to the receiving member at its bottom end. A resilient member, adapted to apply a restoring force to the receiving member when the receiving member tilts, surrounds each receiving member.
In addition, the well known maritime practice of mooring, for holding a vessel in position within an acceptable range of lateral movement, has been used in association with integrated deck installation procedures. U.S. Pat. Nos. 4,648,751 (Coleman), 4,744,697 (Coppens), and 5,037,241 (Vaughn et al.) all describe the mooring of a barge/vessel in relation to the position of an offshore platform substructure such as a jacket.
As indicated by the foregoing, the offshore petroleum industry has sought to alleviate the problems associated with wave-induced motion during mating of an integrated deck structure to a preinstalled jacket structure with an ever-growing array of load transference and alignment devices. While these devices can enable deck setting procedures in periods of relatively fair weather and calm water, such conditions often do not coincide with an optimum project completion schedule, and the costs associated with such devices can be substantial. In addition, mooring methods as currently utilized do not alleviate the shock load damage potential of current deck installation methods. Therefore, a need exists for a deck setting procedure operable in inclement weather and rough seas.